The invention relates to imaging of an original image on a projection area and, more particularly, but not exclusively, to a projector with automatic focusing by way of distance measurement according to the preamble of claim 1 and a corresponding imaging process according to the preamble of claim 12.
Projectors are used to project any type of image onto a surface. The original image can be an optical pattern, for example in form of a slide, or it can be stored in electronic form and transformed by corresponding electrooptic converters into an optical signal. In any event, the image must be represented on the projection area with a predetermined sharpness. This is accomplished by adapting the focus of the projection optics. To be able to adapt the focus, the distance between the projector and the projection area is measured.
A projector with an imaging system is known from US 2005/001999 A1, which enables precise focusing of a projector by using a photo detector. The projector includes an imaging system, with the focusing being performed dependent on the output signal of a photo detector. The photo detector captures the light backreflected from the imaging surface. The optical axis of the imaging system is displaced in relation to a reference axis. The reference axis extends from the center of an original optical image to the center of the projected image. The photo detector captures the light in its capture region proximate to the optical axis, where the position of the optical axis of the optical imaging system on the imaging surface is also located.
The distance is measured by recording the light-dark transition of the lower image edge with a phase comparison sensor. Due to the geometric distance between the two optical axes of the phase comparison sensor, the image edge of each partial unit appears with a different angle depending on the distance. Based on this angle and the width of the base, a triangle is spanned between the optical axes, with the distance being determined by triangulation.
With this state-of-the-art system, a light-dark transition must necessarily exist in the observed region, which is not the case with predominantly dark images or image edge characteristics of the information to be projected that is different from the image aspect ratio of the projector. Additionally, this measurement principle requires a large installation space.
DE 60 2005 000296 T2 discloses a projector with automatic focusing. The projector includes a test structure projection unit for imaging on a projection area a test structure with two regions of different optical density. A focus adjustment unit moves the focal point of the projector, an imaging unit images the projection area, a focus adjustment unit focuses the focal point as a function of an indicator value which is a function of the focusing state.
In other words, by recording the structure with an image recording system, the quality of the actual focus position is measured and the position of the objective is changed until the image shows the best possible “sharpness.”
With this conventional approach, a test pattern must disadvantageously be projected which may be objectionable to the user. In addition, the direction of the displacement of the objective is initially not known, so that the image may initially become less sharp.
JP 2006 184569 A1 describes an imaging device with a focusing mechanism. Depending on the output signal of a line sensor, on the distance of the focal point of a coupled range finder lens and on a base length which corresponds to a distance between the optical axis of an imaging lens and the optical axis of the coupled range finder lens, a distance between the imaging reference point of the imaging lens and the point at which the optical imaging axis on a projection area and a projection surface intersect, is computed with a control device by triangulation. Depending on the computed distance, a motor control signal for a motor is generated, which is used to adjust the rotation position of a focusing ring, thereby focusing the imaging lens.
With this system, test patterns based on lines are projected through the projection optics on the projection area. The test image composed of a line can be recorded with a recording objective arranged with an offset and having a line of light-sensitive photo elements as sensors. The distance to the projection area is measured by triangulation. At the same time, trapezoidal distortions which occur when a projection area is arranged at an angle to the optical axis of the projection can be corrected.
DE 10 2005 034 990 A1 discloses a device for auto-focusing with a projector for imaging at least one original image on a projection area with a projection optics having an adjustable focus for imaging the original image on the projection area, and a camera device for imaging the projection area and generating a projection area image, and with a comparison device for comparing the original image with the projection area image and for adjusting the focus of the projection optics depending on this comparison. However, DE 10 2005 034 990 A1 likewise uses a test pattern consisting of bars for performing auto-focusing. In particular, the position of the projected image in the projection area image is not directly used as a measure for adjusting the focus of the projection optics, but rather the overall sharpness of the image test pattern.
US 2007/0242233 A1 discloses a projector and a projection screen, wherein the projection screen has one or more markers for compensating image distortion. US 2007/0242233 A1 also uses a test pattern for performing auto-focusing.
In the aforedescribed state-of-the-art projectors, a test pattern is disadvantageously required for performing focusing, which may be viewed as being annoying in continuous operation.
US 2008/0024738 A1 discloses a projector with integrated camera, wherein the projector lens and the camera lens are controlled by the same drive to reduce the number of components. Although US 2008/0024738 A1 does not employ a test pattern for auto-focusing, but instead the actual images to be projected, auto-focusing is realized by computing the contrast ratio in the projection area image, which disadvantageously increases the required computing complexity.
It is an object of the present invention to obviate the aforementioned disadvantages of the state-of-the-art and, more particularly, to enable precise focusing of the projector without requiring special calibration projections, which may disturb the projection process.
This object is solved by the projector for imaging an original image on a projection area according to claim 1 and the corresponding imaging method according to claim 12. Preferred embodiments of the invention are recited in the respective dependent claims.
The solution of the object according to the invention is based essentially on the determination of a measurement value for automatic focusing. To this end, a camera module is positioned with a lateral offset relative to the optical axis of the projection. The camera module is oriented so as to be capable to record at least a portion of the projection, ideally the entire projection. In addition, a temporary copy of the content to be projected is generated within the projector. By finding the content to be projected in the camera image and determining the lateral position, the distance to the projection area can be determined by triangulation.
The projector according to the invention for imaging an original image on a projection area includes: projection optics with an adjustable focus for imaging the original image onto the projection area and a camera device for imaging the projection area and for producing a projection area image; a comparison device for comparing the original image (i.e., the image data of the image to be projected) with the projection area image; means for adjusting the focus of the projection optics depending on this comparison, wherein the comparison device is configured to determine the position of the original image (i.e., the image data of the image to be projected) in the projection area image, wherein the position of the original image in the projection area image is used as a measure for determining a projection distance corresponding to the distance between the projector and the projection area and thereby to determine the focus of the projection optics.
Preferably, the position of the original image in the projection area image is used as a direct measure for determining a projection distance corresponding to the distance between the projector and the projection area. Preferably, the projection distance is determined by triangulation using the position of the original image in the projection area image.
Preferably, the offset of the image center of the projection of the original image to be projected from the image center of the recorded image (projection area image) in pixels is used as a measure for determining a projection distance corresponding to the distance between the projector and the projection area. Preferably, the projection distance L is determined from this offset by triangulation.
Preferably, the projection distance L is computed as follows:
  L  =            b      *      R              k      *      2      *              tan        ⁡                  (                      γ            /            2                    )                    wherein b is the distance of the optical axis of the camera from the (parallel) optical axis of the projection optics, γ is the field angle of the camera system, k is the offset of the image center (of the projection) of the original image to be projected in the recorded image from the image center of the recorded image (projection area image) in pixels, R is the resolution of the camera image in the respective direction (preferably in pixels).
To simplify the determination of the image position, the position value k, which represents the offset of the image centers of the projection area image from the image center of the projection in the projection area image, can be transformed into more computationally-friendly values, for example in relation to an offset of an outside edge of the projection area image from the image center of the projection in the projection area image.
Preferred embodiments of the projector according to the invention include as an additional feature or—if the eagerly feasible and advantageous—as combination of additional features, that                an optoelectronic converter is provided for converting the original image into a photographic pattern for an electronic signal;        an optoelectronic converter is provided for converting the original image stored as an electronic data set into an optical image;        a scaling device is provided for matching the size of the original image to the projection image;        the original image and the projection image each have a predetermined number of horizontal and vertical pixels, and the comparison between original image and projection image is made on a pixel-by-pixel basis;        the comparison device displaces the original image and the projection area image relative to each other by a predetermined displacement distance and performs for each displacement distance a comparison to determine an optical overlap between the original image and the projection image;        the displacement distance corresponds to a predetermined number of pixels in the horizontal direction and/or a predetermined number of pixels in the vertical direction;        to compare the original image with the projection area image, a comparison signal is generated as a function of a brightness difference between the original image and the projection area image;        to compare the original image with the projection area image, a comparison signal is generated as a function of a color difference between the original image and the projection area image;        a feature extraction device for extracting at least one image feature from the projection area image is provided, wherein the comparison between original image and projection area image is performed essentially only based on the at least one image feature;        a temperature sensor for measuring a temperature of the optical components of the projection optics and/or of the camera device and a correction element for correcting the comparison signal as a function of the temperature are provided.        
The corresponding imaging method according to the invention for imaging an original image on a projection area with the projector, has the following steps: imaging the original image on the projection area with a projection optics having an adjustable image width and imaging the projection area and generating a projection area image with a camera device, and is characterized by comparing the original image with the projection area image and adjusting the image width of the projection optics with a comparison device as a function of the comparison.
Preferred embodiments of the imaging method according to the invention include as an additional feature or—if technically feasible and advantageous—as a combination of additional features,                that the original image is converted with an optoelectronic converter from a photographic pattern into an electronic signal, that the original image stored as an electronic data set is converted by an electro-optic converter into an optical signal; matching the size of the original image and of the projection image with a scaling device;        displacing the original image and the projection area image with respect to one another by a predetermined displacement distance and performing the comparison with the comparison device for each displacement distance so as to determine an optimal overlap between the original image and the projection image;        that the displacement distance corresponds to a predetermined number of pixels in the horizontal direction and/or a predetermined number of pixels in the vertical direction;        that for comparing the original image with the projection area image, a comparison signal is generated as a function of a brightness difference between the original image and the projection area image;        that for comparing the original image was a projection area image, a comparison signal is generated as a function of a color difference between the original image and the projection area image;        extracting at least one image feature from the projection area image with a feature extraction device, wherein the comparison between the original image and the projection area image is essentially performed only based on the at least one image feature;        measuring a temperature of the optical components of the projection optics and/or of the camera device with a temperature sensor, and correcting the comparison signal with a correction element as a function of the temperature.        
The solution according to the invention has furthermore the following advantages. Unlike in the state of the art, it is not necessary to superimpose test images which may annoy the viewer. In addition, the distance can be measured and the objective can be specifically moved to the correct position even while projecting unsharp or moving pictures, without the system beginning to “pump.”
Additional features and advantages of the invention can be inferred from the following description of particularly preferred embodiments, wherein reference is made to the appended drawing.
The representation in the Figures is not to scale, and identical elements or elements performing an identical function have identical reference symbols, unless noted otherwise.